Liquid crystals are applied to display media in which the reversible movement of liquid crystal molecules is made use of, such as display devices typically of TN or STN mode. Besides the application to display media, liquid crystals have been studied for applicability to optically anisotropic elements, such as a retardation film, a polarizer, a polarizing prism, a luminance-improving film, a low pass filter, and various optical filters, taking advantage of their anisotropy in physical properties, such as refractive index, dielectric constant, magnetic susceptibility, elastic modulus, and thermal expansion coefficient, as well as their aligning properties.
The above described optically anisotropic elements are obtained by, for example, uniformly aligning the molecules of a liquid crystal compound having a polymerizable functional group or a polymerizable composition containing the liquid crystal compound into a liquid crystal state and irradiating the compound or the composition while being in the liquid crystal state with energy rays, such as ultraviolet rays, to cause photopolymerization. It is required to fix the aligned state of the liquid crystal compound uniformly and semi-permanently.
When the polymerizable composition has a high liquid crystal phase temperature, photopolymerization induced by energy rays may be unintentionally accompanied by thermal polymerization, which will disturb the uniform alignment of the liquid crystal molecules, making it difficult to fix a desired state of alignment. In order to facilitate temperature control during cure, a polymerizable composition showing a liquid crystal phase at or near room temperature has been demanded.
A polymer film is obtained by polymerizing a polymerizable composition in the form of coating film applied to a substrate. If the composition contains a non-polymerizable compound, the resulting polymer film may have insufficient strength or contain residual stress-induced strain. Removing a non-polymerizable compound using, e.g., a solvent can result in a failure to retain film homogeneity and cause unevenness. To obtain a polymer film with a uniform thickness, it is therefore preferred to apply a polymerizable composition in the form of a solution in a solvent to a substrate. Hence, it is desirable for a liquid crystal compound or a polymerizable composition containing it to have good solubility in a solvent.
The above discussed optically anisotropic element is usually used not as a single film but as stacked on a substrate, such as glass, together with other functional material and is therefore required to have solvent resistance and high hardness so as not to interfere with applying the other functional material.
Polymerizable compounds having a (meth)acrylic group as a polymerizable functional group have been extensively studied for use as a monomer providing an optically anisotropic element because they exhibit high polymerization reactivity and produce highly transparent polymers. Inter alia, bifunctional or trifunctional monomers having two or three (meth)acrylic groups are known to be an effective means for providing polymers with improved heat resistance and solvent resistance. For example, patent documents 1 to 6 below propose trifunctional monomers. However, the problem with these monomers is that the resulting polymers can suffer from crystallization or be difficult to control to provide uniform molecular alignment.    Patent document 1: JP 2000-178233A    Patent document 2: JP 2004-043710A    Patent document 3: JP 2003-321430A    Patent document 4: JP 2004-059772A    Patent document 5: JP 2005-309255A    Patent document 6: WO2006/049111